Battery driven d.c. to d.c. converters are known and typically comprise an oscillator driven by a battery which feeds a relatively low voltage oscillatory signal to a primary winding of a transformer, so as to induce in a secondary winding of the transformer, a relatively high voltage oscillatory signal. This high voltage signal is rectified and smoothed to produce a high voltage output for application to a load. The converter may operate utilising the known flyback principle by which the low voltage oscillatory current is arranged to increase steadily in the primary winding and is then abruptly switched off so as to induce a high voltage pulse in the secondary winding when the primary winding current is switched off. Typically a voltage pulse of several kilovolts is induced in the secondary winding from for example a six volt battery supply.
It is desirable to include in the converter a control loop in order to control the high voltage output in the event of fluctuations of battery voltage or fluctuations in the impedance presented by the load. It would be possible to control the output voltage by sensing the output voltage directly and controlling the oscillator in dependence on the sensed voltage. However, because the output voltage may be several kilovolts in magnitude, the sensing could only be performed after first dropping the output voltage through resistors or like elements, with the consequent disadvantage of consuming battery power and reducing the efficiency of the converter. Such efficiency considerations are important where electrical apparatus is to be powered by the converter in a remote location where battery recharging or replacement cannot be achieved readily.